"N Cook" wrote in message
...
Non-technical owner had service manual and started fiddling.
He managed to somehow destroy all active bits in the power supply.
That's corrected but now an intermittent problem.
Main crystal osc. locked on f, all 3 drives to platter are working
and once settled at 33 or 45 RPM it all stays locked into speed strobe.
Always from start or from 33 to 45 locks in quickly but sometimes , about
1 in 3 times going from 45 to 33 will take 2 to 4 seconds to lock in to
lower speed.
Rarely will the brake properly fuction ie stop
between 120 and 270 degrees after stop signal at zero degrees, usually
continues jerkily until the power cut ,time-out, after a few seconds.
Makes no difference to jerking what setting of the brake adjustment pot.
Adding the heavy rubber platter mat does not damp things and improve
matters
thinking some servo gain/resonance effect.



Seems a problem with the section of the custom IC that outputs the braking
signal to the drive IC. The main f oscillator section is fine.
This errant section gates the 33/45 switch and start/stop switch.
Should output 1.5V in run and 0V on braking but in the non-locking
45 to 33 changeover and lack of braking situation it is outputting 3 to 4
volts..
Will try a 1.5V zener and a few K to at least limit.